Splitup seeking handling from mousemove handler

[gkatsev]

This is a follow-up from #7527. That PR is a quickfix to us not marking a click-to-seek as scrubbing so that we won't use fastSeek for those and only use fastSeek when we're actually scrubbing.
Ideally, we'd want to pull out the seeking logic out of the mousemove handler and have both mousedown and mousemove handlers call the seeking logic.
Doing so would also affect the Slider, which means that it'll affect the volume bar as well, so, we'd need to make sure that's all set up properly.

[stale]

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